The present invention concerns calibration of printers and other devices that perform printing and pertains particularly to improved calibration of printing devices.
During the course of operation of a printer, the print parameters which control printing density and shades can drift. It is desirable therefore to perform calibrations to restore print parameters to a desired state.
Calibration can be performed by printing a test pattern on print media. A densitometer or other device is used to sense the amount of toner present on the media. The data from the densitometer can then be used to set printing parameters.
The on-media calibration described above can be used to very accurately set print parameters. However, there are costs associated with the accuracy. Particularly, each calibration cycle consumes media. The calibration cycle is disruptive to customer print jobs, and can reasonably only be done between print jobs. Because of these costs (time, money, materials, user involvement), it is impractical to perform on-media calibration automatically or often. This limits the practicality of on-media calibration to systems that are inherently stable, and where significant process drift occurs only over long periods of time or printing.
One alternative to on-media calibration is off-media calibration. In off-media calibration, calibration is performed by printing a test pattern, not on media usually used for printing, but on a transport belt or some other permanent entity within the printer. In this case, a densitometer or some other device is used to sense the amount of toner present. The data from the densitometer can then be used to set printing parameters.
Off media calibration has the advantage of not consuming media. Off-media calibration can also be performed automatically, with little impact on users. However, off-media calibration is prone to inaccuracy since the measurement is made in a substitute fashion (e.g. printing on the transport belt) or at some intermediate stage in the printing process. It is necessary, therefore, in order to adjust print parameters during off-media calibration, to rely on correlations made at the factory to determine how density values detected during off-media calibration relates to printing density present when printing is actually performed on-media. A user of the printer may be able to adjust correlation values set at the factory; however, this typically requires that a test page is printed and viewed by the user. The user then makes some judgements judgments and enters adjustment information back into the printer. Several iterations may be required before a user is content with the adjustments made to the correlation values. The adjustment factors provided in any such calibration method must also be predetermined in the factory, and may not be sufficient to compensate for actual variations.